1. Field of the Invention
The present invention relates to a system where a communication apparatus communicates with a communication partner through a relay apparatus.
2. Description of the Related Art
Network address translation (NAT) and network address port translation (NAPT) are technologies for translation between an Internet Protocol (IP) address (private address) used within a private network and IP address (global address) used outside the private network.
So far, various methods have been discussed that enable an apparatus incorporated in a private network to obtain a global address translated by the NAT.
For example, a method has been discussed that uses a Simple Traversal of User Datagram Protocol (UDP) through NATs (STUN) server. In the method, an apparatus in a private network sends a request for a global address to a STUN server outside the private network, and the STUN server sends a global address to the apparatus as required.
Japanese Patent Application Laid-Open No. 2004-15685 discusses a method in which a communication apparatus generates an electronic mail to be sent to the apparatus itself, and transmits the mail to a mail server outside a private network, so that a global address is acquitted out of header information of the mail transmitted from the mail server to the communication apparatus.
In the case, however, where a global address is acquired using an external server (e.g., a STUN server or a mail server), storage of an IP address of the external server in advance is required. In addition, if the IP address is changed or the external server is halted, the target global address cannot be acquired.